bpo-40082: trip_signal() uses the main interpreter (GH-19441)

bpo-40082: trip_signal() uses the main interpreter (GH-19441)

Fix the signal handler: it now always uses the main interpreter,
rather than trying to get the current Python thread state.

The following function now accepts an interpreter, instead of a
Python thread state:

* _PyEval_SignalReceived()
* _Py_ThreadCanHandleSignals()
* _PyEval_AddPendingCall()
* COMPUTE_EVAL_BREAKER()
* SET_GIL_DROP_REQUEST(), RESET_GIL_DROP_REQUEST()
* SIGNAL_PENDING_CALLS(), UNSIGNAL_PENDING_CALLS()
* SIGNAL_PENDING_SIGNALS(), UNSIGNAL_PENDING_SIGNALS()
* SIGNAL_ASYNC_EXC(), UNSIGNAL_ASYNC_EXC()

Py_AddPendingCall() now uses the main interpreter if it fails to the
current Python thread state.

Convert _PyThreadState_GET() and PyInterpreterState_GET_UNSAFE()
macros to static inline functions.